1. Field of the Invention
The present invention relates to a battery and a method for preparing the same. More particularly, the present invention relates to a battery obtained by using an electrode whose resistance becomes larger in accordance with temperature increase.
2. Description of the Background
Recently, with development in electronic appliances, high leveling of capacity and output density of a battery used as a power source is being advanced. As a battery, which can satisfy these requirements, attention is paid to a lithium ion secondary battery. The lithium ion secondary battery has an advantageous effect that energy density is high, while a sufficient counterplan for safety is required because a non-aqueous electrolytic solution is used.
As a counterplan for safety it has been conventionally suggested to incorporate a safety valve which releases increased internal pressure, or a PTC device of which resistance increases in accordance with the heat generated from external short circuit to break an electric current.
For example, as disclosed in Japanese Unexamined Patent Publication No. 328278/1992, there is known a method for attaching a safety valve and a PTC device to the positive electrode cap of a cylindrical battery. However, when the safety valve is operated, water in air may invade into the battery to react with lithium in the negative electrode and there is a fear of an exothermic reaction.
On the other hand, the PTC device successively breaks external short-circuit without causing any troubles. As a safety component running firstly at the emergency of the battery, the PTC device can be designed to run when the battery reaches at least 90xc2x0 C. due to external short circuit.
Since the conventional lithium secondary battery has the structure mentioned above, there exist the following problems.
At occurrence of short-circuit and temperature rise inside the lithium secondary battery, increase of the short-circuit current can not be controlled in a conventional lithium secondary battery.
When the short-circuit inside the lithium secondary battery increases a temperature, a polyethylene or polypropylene separator interposed between the positive electrode and the negative electrode is expected to have a function that the separator softens or melts to close holes thereon and release or seal a non-aqueous electrolyte contained in the separator to decrease its ion conductivity, and thereby reducing the short-circuit current.
But a separator away from the heating part does not always melt. Also, when a temperature further rises, the separator melts and is fluidized, and thereby the function to electrically insulate the positive electrode and the negative electrode is lost to cause short-circuit.
Besides, particularly in a lithium ion secondary battery, a negative electrode is formed by applying a slurry comprising a negative electrode active material such as graphite, a binder such as PVDF (poly(vinylidene fluoride)) and a solvent, onto a substrate such as a copper foil which formus a collector, and drying it to form a thin film thereof. A positive electrode is also formed as a thin film in the same manner onto a substrate such as an aluminum foil, which forms a current collector. The positive electrode contains a positive electrode active material such as LiCoO2, a binder and a conductive agent.
The conductive agent is used to increase an electronic conductivity at a positive electrode when the positive electrode active material has insufficient electronic conductivity. As the conductive agent, there is used carbon black (such as acetylene black) or graphite (such as artificial graphite KS-6 available form LONZA Co., Ltd.).
These positive and negative electrodes have a problem that when a temperature of the battery increases to at least 100xc2x0 C. due to internal short-circuit or the like, large short-circuit current is generated and the temperature of the battery further increases due to the generation of heat, leading to a further increase of short-circuit current.
The present invention has been carried out in order to solve the above problems. The object of the present invention, in which an electrode is used of which resistance is increased when the temperature of the battery increases, is to provide a highly safe battery having excellent properties which is capable of controlling increase of short-circuit current even at temperature rise due to generation of heat.
The first battery of the present invention comprises an electrode containing an active material, and an electronically conductive material and a conductive agent contacted to the active material. The above electronically conductive material contains electrically conductive filler and a crystalline resin. The total amount of the above electronically conductive material and the conductive agent is 1 to 20 parts by weight based on 100 parts by weight of the above active material. According to this, the above electronically conductive material contains the electrically conductive filler and the crystalline resin to increase resistance thereof with temperature rise, and thus increase of current at electrode can be controlled with temperature rise. Furthermore, since the amount of the conductive agent was set to 1 to 20 parts by weight based on 100 arts by weight of the electronically conductive material, resistance at the electrode can be lowered to improve battery characteristics, and short-circuit current can be controlled to a low value.
The second battery of the present invention is that 0.5 to 30 parts by weight of the conductive agent is contained based on 100 parts by weight of the electronically conductive material. According to this, the electrode has low resistance before increase of changing ratio of resistance, since contains 1 to 30 parts by weight of the electronically conductive material based on 100 parts by weight of the active material.
The third battery of the present invention is that the average particle size of the conductive agent is {fraction (1/1000)} to {fraction (1/10)} based on the average particle size of the electronically conductive material. According to this, since the average particle size of the conductive agent is {fraction (1/1000)} to {fraction (1/10)} compared to that of the electronically conductive material in the seventh battery, current inside electrode is efficiently collected to improve battery characteristics.
The fourth battery of the present invention comprises an electrode having an active material layer containing an active material, and an electronically conductive material and a conductive agent contacted to the active material, wherein the above electronically conductive material contains an electrically conductive filler and a crystalline resin and wherein the active material layer comprises two layers which has high ratio of the electronically conductive material and which has low ratio thereof. According to this, electrodes having the active material layer comprising two layers one of which has high ratio of the electronically conductive material and the other of which has low ratio of the electronically conductive material can make a battery to have very low short-circuit current, high discharging capacitance and excellent battery properties such as cycle life.
The fifth battery of the present invention is that a melting point of the crystalline resin in the electronically conductive material is in a range of 90xc2x0 C. to 160xc2x0 C. According to this, since the electronically conductive material contains the crystalline resin having a melting point in the range of 90xc2x0 C. to 160xc2x0 C. to increase in changing ratio of resistance at about 90xc2x0 C. to 160xc2x0 C., characteristics of the battery and safety can be coexistent with each other.
The sixth battery of the present invention is that an amount of the electrically conductive filler is 40 to 70 parts by weight in the electronically conductive material. According to this, by setting the amount of the electrically conductive filler to 40 to 70 parts by weight in the electronically conductive material, changing ratio of resistance of the electrode can be large at a pre-determined temperature and discharging capacitance of the battery can be increased when the electrode is applied to the battery.
The seventh battery of the present invention is that a carbon material or an electrically conductive non-oxide is used as the electrically conductive filler. According to this, since the carbon material or the electrically conductive non-oxide is used as the electrically conductive filler, the electric conductivity of the electrode can be improved.